The revolutionary discovery by Chu and his collaborators of superconductivity at about 90.degree. K. in the yttrium, barium, copper oxide system has led to intense research efforts to prepare, characterize and fabricate a variety of related materials for possible future applications.
This invention relates to superconductive metal oxide ceramics, in particular to YB.delta..sub.2 Cu.sub.3 O.sub.x, wherein x equals from 6.5 to 7.2.
The technology of superconductivity, particularly metal oxide ceramics which conduct electricity with no resistance at temperatures above the boiling point of liquid nitrogen, unlike previously known materials that can superconduct only near absolute zero, has been rapidly developing. These materials, i.e. certain metal-oxide ceramics, can conduct electricity with no resistance at temperatures above the boiling point of liquid nitrogen (77.degree. K. or -196.degree. C.). The discovery of these materials has been quite recent, and the demand for practical application of these materials will be ever increasing in the future. In particular, it is desirable to develop these new metal-oxide ceramics, particularly YBa.sub.2 Cu.sub.3 O.sub.x, wherein x equals from 6.5 to 7.2 into useful conductor shapes such as wires, films, or thin sheets.
The superconducting phase in the yttrium barium copper oxide system has been determined to have the composition YBa.sub.2 Cu.sub.3 O.sub.9-.delta. (where .delta.=2.1.+-.0.05). This material exhibits tetragonal symmetry at high temperature but transforms to orthorhombic symmetry when cooled slowly in oxygen atmosphere. It has been shown that superconducting phase is this orthorhombic phase.
The most common method for preparing this composition has involved mixing yttrium oxide (Y.sub.2 O.sub.3) copper oxide (CuO) and barium carbonate (BaCO.sub.3) followed by heating to about 750.degree. C. to 1000.degree. C. for times ranging between 6 to 16 hours, in a static air or flowing oxygen atmosphere, with several intermediate grindings or ball millings to facilitate a reaction between these starting materials to form the desired ceramic composition. This method of solid state reaction of the oxides and carbonates has been utilized by many investigators in the synthesis of YBa.sub.2 Cu.sub.3 O.sub.9-.delta..
Although solid state reactions result in the superconducting phase; the method has several shortcomings. Generally the product contains a mixture of phases, may even contain chemical impurities introduced during the intermediate grinding or ball milling operations, and finally it is a rather tedious and time consuming method. One of the serious problems in the method described above is the slow decomposition of BaCO.sub.3 which results in formation of barium deficient oxide phases rather than pure superconducting phase. Also, the elimination of residual carbonates is difficult.
Precipitation of the cations of yttrium, barium and copper in the proper ratio as oxalates and carbonates followed by a high temperature heat treatment to decompose into corresponding oxide has also been the subject of experiments but no significant improvement in the process was apparent as the oxalates and carbonates do not decompose readily and completely to yield pure, single phase superconducting oxide.
Accordingly, from the foregoing discussion it can be seen that there is a need for a new method for preparing a chemically pure and preferably single phase superconducting oxide material having yttrium, barium and copper oxides in the proper ratio in order to form a superconducting ceramic.
In particular, there is a need for a new method which avoids the solid state (particularly carbonate) reaction phases of the prior art, but which at the same time does not involve significant solution phases which must be boiled, evaporated, etc. In short, there is a need to have a technique for preparation of superconductor ceramics which does not use a slow, solid phase reaction and which avoids the cost ineffective problems of solution reactions wherein large amounts of water are employed.
Another primary objective of the present invention is to develop a method of preparing yttrium, barium and copper oxide superconductor ceramics which have the right stoichiometry, wherein the process is cost effective, and wherein the process is amenable to a large scale production with the reaction occurring in a single phase melt.